1. Field of the Invention
The present invention relates to active power line conditioners which are utilized to regulate the quality of electrical energy delivered from an electrical energy source to an electrical load. More particularly, the invention relates to a series-parallel active power line conditioner (APLC) which has enhanced peak voltage regulation capability by functioning to temporarily increase DC link energy during excessive variations of the AC line voltage.
2. Description of the Prior Art
Electric supply networks are increasingly being exposed to the consequences of nonlinear loads, such as data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical apparatus and communication equipment. Such loads draw nonlinear pulse-like currents instead of sinusoidal currents drawn by linear loads (i.e., resistors, inductors and capacitors). These nonlinear currents flow through the source impedance of the electrical energy source, thus causing distortion of the AC line voltage.
This voltage distortion may produce a number of undesired effects. For example, sensitive loads connected to the network may experience operational difficulties. Additionally, the RMS current supplied by the electrical energy source will generally increase due to the presence of harmonics in the pulse-like currents. These harmonic currents may significantly increase I.sup.2 R losses in interposing transformers.
Another problem which may have significant effects on many types of electrical equipment is the occurrence of temporary sags in the AC supply voltage. For example, many types of electrical equipment utilize a power supply input stage which converts the AC line voltage to DC voltage via a full wave rectifier connected across one or more large filter capacitors. In normal operation, the filter capacitor recharges with each peak of the rectified line voltage. It is only during this peak that the load is actually drawing current from the electrical supply network. When the rectified line voltage is lower than the voltage level on the filter capacitors, the rectifier diodes will prevent current from flowing. If, however, the AC line voltage does not maintain an adequate peak-to-peak level, these filter capacitors will not be able to maintain their required peak charge levels. As a result, operation of the associated electrical equipment may be affected.
The effects of many of these problems can be mitigated through the use of power electronic devices known as active power line conditioners. Such devices typically comprise one or two switching inverters arranged in a series, parallel, or series-parallel configuration. The inverters are controlled (generally by pulse width modulation (PWM) techniques) to effect a flow of current between a DC energy storage element and the AC supply lines to which they are connected. Such devices are shown and described in U.S. Pat. Nos. 4,651,265 and 3,825,815, which are incorporated herein by reference.
When a single inverter is used, this current may consist of the harmonic and ripple currents required by the load. In a series-parallel configuration, two inverters are arranged to share a common DC link. In this arrangement, the inverters may cooperate to effect a transfer of real power between the source or load and the DC link. This may be helpful to insure that the load continually sees ideal current and voltage characteristics.
The series inverter in a series-parallel active power line conditioner is typically coupled to the AC supply line via a coupling transformer. The series inverter applies a voltage of selected magnitude and polarity to the secondary winding of the transformer, which produces an AC regulation voltage on the primary winding of the transformer. This AC regulation voltage bucks or boosts (i.e., is added to or is subtracted from) the AC supply voltage to maintain the AC output voltage seen by the load at a regulated nominal value. The parallel inverter may operate as an active filter to supply real and reactive currents to the load as needed as well as cooperating with the series inverter to effect real power transfer.
The voltage regulation capability of a series-parallel active power line conditioner is given in terms of a rated buck/boost voltage range. If the line voltage magnitude stays within the rated buck/boost range, the output voltage stays essentially constant and sinusoidal at the nominal value. Typically, the rated buck/boost range of an active power line conditioner is expressed as a percentage of the nominal output voltage. Generally, this buck/boost range is selected to fall between .+-.10% to .+-.25% of the nominal output voltage. (The voltage buck/boost range tends to be symmetrical due to symmetry of the series inverter power circuit.)
When the supply voltage sinks below the rated boost range of the active power line conditioner, however, the output voltage tends to also be dragged down. Thus, the line voltage seen by the various loads connected to the electrical power network will fall below the nominal value. As a result, the quality of power delivered to these loads is no longer insured. This is particularly true in the case of loads having rectified capacitive inputs, since the filter capacitors may not be able to obtain their peak charge level. Additionally, transient voltage sags or surges may occasionally exceed the selected rating of the series voltage regulator.
In order to provide the greatest assurance of power quality to loads supported by the APLC, it is therefore desirable for the device to have as large buck/boost regulation range as is practical. The weight and cost, however, associated with the magnitude of voltage regulation capability is proportional to this regulation range. As such, an active power line conditioner with a higher regulation range would be expected to be larger and more expensive than a similar device with a smaller regulation range. This may tend to negatively affect the commercial viability of such equipment. Additionally, the installation of a larger coupling transformer to support excessive input voltage sags would have double iron losses that would adversely effect the overall efficiency.